Substrate-Surface Effect on Initial Growth Process of Microcrystalline Silicon Films
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743 Mat. Res. Soc. Symp. Proc. Vol. 452 0 1997 Materials Research Society
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Fig. 1 Typical STM images of the surface of H-terminated Si(l 11) observed just after a wetchemical treatment. In Fig. l(a), 400 x 400 nm 2 topography, monolayer-height steps can be seen. In Fig. 1(b), 14 nm x 14 nm 2 topography, H-terminated Si(1 11)-1 x 1 structure is observed. SiO2 -terminated surface was prepared by exposing the H-Si(1 11) surface to 02 plasma in the loadlock chamber [18]. Oxidation conditions were as following: a source gas was 02 (0.1 seem) diluted by He (50 sccm), rf power and total gas pressure were 20 W and 100 mTorr, respectively. 02 plasma was exposed to the sample at room temperature for 15 s. The formation of silicon oxide layers was confirmed by AES. Thickness of the silicon oxide layer was estimated to be 0.5 nm from the lineshape of SiLvv (92 eV) Auger features. The plasma-oxidized H-Si(1 11) surface will be called SiO 2/Si( 11) in the following text. Deposition of hydrogenated silicon films was carried out on the H-Si(1 11) and SiO2 /Si(l 11) surfaces by plasma-enhanced CVD using 13.56 MHz2 rf excitation of Sil 4 (1 sccm) diluted by H 2 (99 seem). The rf power density was 0.42 W/crm. The triode-type configuration was used, where a negative bias of - 50 V was applied to the third electrode placed at 3 cm below of the power electrode and 2 cm above the grounded electrode (the sample holder). The substrate temperature was 350 'C. In all experiments, uncontrolled deposition during the plasma ignition and stabilization was prevented by a mechanical shutter located above the substrate. After depositions, the samples were transferred through a gate valve to an ultra-high vacuum chamber (less than 10-10 Torr), where STM observation was performed. The conditions of STM measurements are as following: the tunneling current is 200 pA, the sample voltages are 2.0 V before oxidation and larger than 3.5 V after oxidation. RESULTS AND DISCUSSIONS Initial Surfaces Figure 1 shows STM topographies of the surface of H-Si(l 11) observed just after a wetchemical treatment. In Fig. 1(a), a 400 x 400 nm 2 image, atomically flat terraces with about 100 rnm in width can be seen along with several monolayer-height steps. There are triangular etch pits with a lateral dimension up to a few hundreds nm. The flat terraces are terminated by silicon mono-hydride bondings, as shown in Fig. 1(b). Bright spots are arranged in a separation of 0.39 nm. This is known as a H-terminated Si( 11I)-I x 1 structure, which reproduced the results of the previous reports on the STM observation of similarly-prepared Si(1 11) surfaces [19]. Holes with a lateral size of around several nms can be seen among a mono-hydride regular lattice.
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Fig. 2 Typical STM images of the surface of an ultra-thin SiO2 layer prepared on H-terminated Si(1 11) by O2 remote plasma. The triangular holes and monolayer-height steps observed in Fig. 1(a) are conserved even after the oxide formation. Figure 2 shows a typical, 400 x 400 nm2 , STM topography of S
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